Recently, a Magnetic Random Access Memory (hereinafter abbreviated to MRAM) in which a Tunneling Magneto Resistive (hereinafter abbreviated to TMR) effect is utilized is developed as a nonvolatile semiconductor memory (semiconductor memory device). The MRAM is a nonvolatile semiconductor memory having features such as high-speed write/read, a low-power-consumption operation, and a high density memory storage, and expected to be applied as a working memory. The MRAM includes a Magnetic Tunnel Junction (hereinafter abbreviated to MTJ) element, and the MTJ element is a magneto-resistance element having a large magneto-resistance change value.
Particularly, the MTJ element basically adopts a three-layer stacked structure including a storage layer made of a magnetic film in which data is stored by changing a magnetization direction, a reference layer made of magnetic films that is used to unidirectionally fixed magnetization, and a tunnel junction layer (non-magnetic layer) that is made of an insulating film and formed between the storage layer and the reference layer. When a current is passed through the MTJ element including storage layer/tunnel junction layer/reference layer, a resistance value of the MTJ element is changed according to the magnetization direction of the storage layer relative to the magnetization direction of the reference layer. Specifically, the resistance value takes a minimum value when the magnetization directions of the storage layer and the reference layer are parallel to each other, and the resistance value takes a maximum value when the magnetization directions are antiparallel. The phenomenon is called a Tunneling Magneto-Resistance effect (hereinafter referred to as a TMR effect), and the TMR effect is utilized in a memory operation such that the state in which the magnetization directions of the storage layer and the reference layer are parallel is set to data “0” while the state in which the magnetization directions of the storage layer and the reference layer are antiparallel is set to “1”.
Conventionally, in order to operate the MRAM, a current is passed through a wiring disposed near the storage layer of the MTJ element, and the magnetization direction of the storage layer is inverted by utilizing a magnetic field generated around the storage layer (magnetic field write method). However, in the magnetic field write method, although the generated magnetic field can be increased with increasing current, the current permitted in the wiring is restricted due to the maximum current density based on reliability of the miniaturized MRAM, whereby the large-capacity memory of the MRAM is hardly fabricated. The write current necessary to write the data can be decreased by bringing the wiring closer to the storage layer or by devising a material used for the wiring. However, a coercive field of the storage layer is increased in principle by the miniaturization of the MJT element. That is, in the magnetic field write method, it is difficult to achieve a balance between the miniaturization of the MTJ element and the decrease of the write current.
Therefore, recently a spin injection write method in which the magnetization is inverted by utilizing a spin-polarization current is being investigated. In the spin injection write method, the spin-polarization current is passed through the MTJ element to invert the magnetization direction of the storage layer. In the spin injection write method, the number of spin-polarized electrons necessary for the magnetization inversion is decreased with reducing volume of the storage layer in which the magnetization direction is inverted, which allows the write current to be decreased.
The MTJ element can roughly be classified into two. One is an in-plane magnetization type MTJ element in which an in-plane magnetic film having an easy axis of magnetization parallel to a film surface is used as the magnetic film. The other is a perpendicular magnetization type MTJ element in which a perpendicular magnetic film having the easy axis of magnetization perpendicular to the film surface is used as the magnetic film.